European Patent Application 329884/1989 discloses a method of marking a cable using an intense source of ultraviolet or visible radiation, preferably a laser source. According to this method, the exposure of PTFE, FEP, and ETFE electrical insulating layers containing up to 20 wt %, preferably up to 5 wt %, of TiO.sub.2 as photosensitive substance to the intense radiation results in a darkening where the radiation is incident. By controlling the pattern of incidence, marks such as letters and numerals can be formed.
The darkening of the substrate is understood to be caused primarily by interaction between the radiation and the TiO.sub.2 pigment. When ultraviolet (UV) radiation is used and the polymer matrix is transparent to UV as for PTFE and FEP, this interaction will be the sole contribution to darkening. In some cases, the polymer itself may darken somewhat and contribute to the mark, as for ETFE.
The laser marking technique is attractive for generating identifying marks on insulated wires to be used in complex wire harnesses, such as those found in airplanes. Since the multiplicity of circuits would impose unreasonable demands on color coding and on inventory, and since white is good background for marking, white is favored for such uses. Laser marking has advantages over alternative marking techniques such as hot stamping and ink printing, in that there is no potential for mechanical damage as in hot stamping, the difficulties of adhering inks to fluoropolymers are avoided, and the mark is actually beneath the surface and not easily abraded away.
An important characteristic that contributes to the readability of a mark is the contrast between mark and background. Contrast numerically is the difference between reflectances of mark and background, divided by reflectance of the background, and may be expressed as a number in the range 0-1 or in percent. Maximum possible numerical contrast corresponds to an absolutely black mark (zero reflectance). If the mark is not absolutely black (non-zero reflectance), then the whiteness of the background contributes with numerical contrast increasing as background reflectance approaches unity. Minimum contrast acceptable to aircraft manufacturers for ease of wire identification is about 60% for melt-fabricated fluoropolymer insulation or jacket. While existing compositions comprising TiO.sub.2 pigment in a fluoropolymer matrix have provided generally white appearance, such substrates sometimes yield laser marks with good contrast, but at other times relatively poor contrast. This is illustrated by the photomicrographs of laser marks in FIG. 6. The mark in FIG. 6(b) has low contrast with respect to the background, of the order of 50% and would not meet a minimum contrast requirement of 60%. Improved contrast and uniformity of contrast are desired to facilitate identification of wires by operators during production and maintenance operations on wire harnesses, for example, those used in aircraft electrical systems.
U.S. Pat. No. 5,206,280 discloses the use of a second white pigment that does not absorb UV radiation to enhance background whiteness and thereby enhance the contrast of laser marks. No performance data are given.